This present application relates generally to systems and apparatus for improving the efficiency and/or operation of turbine engines. More specifically, but not by way of limitation, the present application relates to improved systems and apparatus pertaining to compressor operation and, in particular, the efficient reintroduction of leakage flow into the main flow path.
As will be appreciated, the performance of a turbine engine is largely affected by its ability to eliminate or reduce leakage that occurs between stages in both the turbine and compressor sections of the engine. In general, this is caused because of the gaps that exist between rotating and stationary components. More specifically, in the compressor, leakage generally occurs through the cavity that is defined by the shrouds of compressor stator blades, which are stationary, and the rotating barrel that opposes and substantially surrounds the shroud. Flowing from higher pressure to lower, this leakage results in a flow that is in a reverse direction of the flow in the main flow path. That is, the flow enters the shroud cavity from a downstream side of the shroud and flows in an upstream direction where it is discharged back into the main flow from an upstream side of the shroud.
Of course, seals are employed to limit this flow. However, given that one surface is in motion and the other is stationary, conventional seals are unable to prevent much of this leakage flow from occurring. The reduction of the gap between stationary and rotating structures is desirable, but its elimination is usually not practical due to inevitable different thermal characteristics between the rotating and stationary components, as well as the centrifugal characteristics of the rotating components. With the added considerations of component manufacturing tolerances and variation in operating conditions, which govern thermal and centrifugal characteristics, it is generally the case that a leakage gap forms during at least certain operating conditions. Of course, leakage generally results from a pressure difference that exists across a leakage gap. However, while it might be possible to reduce the pressure difference across the leakage gap, this generally comes at too high a price, as it places an undesirable limitation on the aerodynamic design of working fluid velocity components.
It will be appreciated that compressor leakage of this nature decreases the efficiency of the engine in at least two appreciable ways. First, the leakage itself decreases the pressure of the main flow through the compressor and, thus, increases the energy that the engine must expend to raise the pressure of the main flow to desired levels before it is delivered to the combustor. Second, mixing losses occur as the leakage flow exits the shroud cavity and reenters the main flow path.
As one of ordinary skill in the art will appreciate, mixing losses of this type may be significant and result in appreciable losses in compressor efficiency. One reason why mixing losses are relatively high is because, at the point of mixture, the leakage flow and the main flow are flowing in dissimilar directions and/or dissimilar velocities. More particularly, the main flow, having just passed through the rotor blades of the previous stage, flows at a relatively high velocity and with a significant tangential directional component. Whereas, the leakage flow, having negotiated the typically tortured pathway through the shroud cavity, flows at a relatively slow velocity and is directed in a primarily radial direction, and lacks the tangential directional component of the main flow.
As a result, there is a need for improved systems and apparatus that reduce the mixing loses that occur when the leakage flow reenters the main flow of the compressor.